Improve Sender batching logic

[martosaur]

💡 Motivation and Context

Currently, Sender will occasionally attempt to send empty batches and get 400 from the ingestion API. This is happening due to a flaw in batching logic: whenever we receive first event in a batch, we set a timer. But if max batch size is reached, we send the batch immediately. This means that when the timer fires off, current batch can be empty. This PR contains 2 improvements:

1. Whenever we decide to send a batch based on its size, we cancel current timer
2. If the timer still goes off and our current batch is empty, we simply ignore it and do not attempt to send it.

💚 How did you test it?

Unit tests

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran sampo add to generate a changeset file